(1) Field of the Invention
The invention relates to a system for transmitting binary data signals with a given symbol rate 1/T from a transmitter to a receiver via a transmission channel of limited bandwidth, this transmitter comprising a data signal source, a clock signal source for synchronizing the data signal source, a modulation stage comprising a carrier oscillator connected to the data signal source for generating an angle-modulated carrier signal of a substantially constant amplitude and a continuous phase, and an output circuit for supplying the angle-modulated carrier signal to the transmission channel, the receiver comprising an input circuit for deriving the transmitted angle-modulated carrier signal from the transmission channel, a circuit coupled to the input circuit for recovering two reference carriers with a phase difference of .pi./2 rad., a demodulation circuit connected to the reference carrier circuit for coherently demodulating the transmitted angle-modulated carrier signal by these reference carriers for generating first and second demodulated signals, a circuit coupled to the input circuit for recovering two reference clock signals of half the symbol rate 1/(2T) with a phase difference of .pi. rad., and a regeneration circuit comprising two sampling circuits connected to the reference clock signal circuit for sampling the first and second demodulated signals with these reference clock signals and further comprising a logic combination circuit for obtaining regenerated binary data signals from the sampled first and second demodulated signals.
Several modulation methods for efficient data transmission over telephone lines have been developed and introduced these last fifteen years. In almost all cases these modulation methods result in a modulated carrier signal showing amplitude variations, and they utilize linear modulators and amplifiers.
However, these modulation methods are not so suitable for data transmission over radio links because in radio communication systems a high power efficiency requires the use of components having a non-linear amplitude transfer function and the spectrum at the output of such a component, for example a class-C amplifier, will be broader than that at the input if the signal shows amplitude variations. Radio communication systems must therefore utilize modulation methods resulting in a modulated carrier signal of a substantially constant amplitude, which implies the use of angle modulation (frequency or phase modulation).
The increasing need for systems for data transmission over radio links also imposes the requirement on the modulation methods to be utilized there of an efficient use of the bandwidth of the available transmission channel. Even if, to this end, a modulation method is used resulting in an angle-modulated carrier signal having a constant phase, the spectrum of this carrier signal will almost always be broader than that of the equivalent base-band signal. Limiting this spectrum by means of a channel filter is an unattractive technique for radio communication systems, as the practical implementation of such a filter with accurately prescribed amplitude and phase characteristics and, frequently, a very narrow relative bandwidth in the radio frequency range is particularly difficult and because many systems are, in addition, of the multi-channel type in which the carrier frequency to be transmitted must be able to assume a large number of different values. Consequently, in radio communication systems, a possible limitation of the spectrum of the angle-modulated carrier signal must be effected by means of premodulation techniques.
A further requirement of the modulation methods to be used in radio communication systems is that the corresponding detection methods result in an error probability as a function of the signal-to-noise ratio which degrades as little as possible relative to the error probabilty for optimum base-band transmission of the data signals. In addition, the receiver must also be able to detect the data signals reliably if unknown frequency shifts occur between transmitter and receiver. These requirements imply that coherent demodulation must be used in the receiver and that--in view of the required efficient use of power and bandwidth--it must be possible to recover the carrier and clock signal references required in the receiver from the transmitted modulated carrier signal itself.
(2) Description of the Prior Art
A system of the type mentioned in the preamble for transmission of binary data signals over radio links is known from reference D(1) In this system a modulation method is used which is a special case of phase coherent FSK (Frequency-Shift Keying) having a modulation index equal to 0.5 and which is known as FFSK (Fast Frequency-Shift Keying) or as MSK (Minimum-Shift Keying). The FFSK-method results in an angle-modulated carrier signal of a constant amplitude and a continuous phase which linearly increases or decreases during a symbol period T by an amount of .pi./2 rad., depending on the binary value of the relevant data symbol; the binary data signals can be detected optimally by means of orthogonal coherent demodulation and the carrier and block signal references required therefor can be recovered from the transmitted FFSK-signal itself.
Consequently, the FFSK-method has many properties which are desirable for efficient data transmission over radio links. Particularly, the power density spectrum of the FFSK-signal shows lower sidelobes than that of comparable signals obtained by means of conventional modulation methods such as 4-PSK (4 Phase-Shift Keying). However, these spectral sidelobes still cause interference in adjacent transmission channels, which interference has a level that is unacceptable for many practical applications.
As stated above, limiting the FFSK-spectrum by means of a channel filter is an unattractive technique for radio communication systems. In view of the many desired properties of the FFSK-method, much attention has therefore been paid in the past few years to premodulation techniques for further reducing the spectral sidelobes without sacrificing these desirable properties. Reference D(2) discloses a generalization of the FFSK (MSK)-method for reducing the spectral sidelobes by means of suitably chosen pulse shapes for the data symbols. This modulation method is known as SFSK (Sinusoidal Frequency-Shift Keying) and results in an angle-modulated carrier signal of a constant amplitude and a continuous phase, which decreases or increases sinusoidually during a symbol period T with an amount of .pi./2 rad. However, only for frequencies spaced more than twice the symbol frequency 1/T from the carrier frequency, the power density spectrum of this SFSK-signal falls considerably below that of the FFSK-signal, so that the SFSK-method furnishes no improvement as regards the reduction of the most annoying spectral sidelobes.